In the past, Natural Gas-fired Combined Cycle (“NGCC”) systems have been a preferred choice for fossil-fueled electric generation. Recently, as demand and price for natural gas has increased, Integrated Gasification Combined Cycle (“IGCC”) systems have become an economically attractive alternative for power generation. In IGCC systems, a carbonaceous fuel such as coal is converted to a synthetic gas fuel, termed syngas. This gas mixture is formed by partial oxidation of hydrocarbons at elevated temperatures. It predominantly comprises carbon monoxide and hydrogen, also including a variety of other constituents. The composition of syngas can vary considerably, depending on the carbonaceous source and the reaction conditions.
IGCC systems present an opportunity to deploy coal, an abundant resource, in an economical and clean manner, bringing improved efficiency and cost over conventional coal-burning steam turbine power generation as well as NGCC systems. The efficiency advantage in an IGCC system stems primarily from the combined cycle. Hot gas products, resulting from syngas combustion, exiting the gas turbine are used to elevate the temperature of water or steam. The resulting steam then undergoes a conventional Rankine cycle. With a typical gasification efficiency of about 80 percent and combined cycle efficiency (gas turbine and steam turbine) of about 58 percent, it is possible to achieve an overall efficiency of 46.4 percent. By way of comparison, the overall efficiency of a typical coal-burning steam turbine power plant is less than 45 percent.
Often syngas includes sulfides and nitrous oxides which are normally removed from the mixture prior to combustion in order to provide an environmentally clean exhaust gas from the combustion stage. To effect this removal, the syngas is sometimes cooled to near room temperature with heat recovery steam generators, so that sensible heat in the high temperature syngas is recovered by a heat exchange process, such as conversion of water into steam. Nonetheless, the form of syngas that enters the gas turbine is a toxic mixture. When used in a combined cycle system there may be concern that syngas will enter subsequent stages of the power system and cause risks of explosion or that toxic constituents may be introduced to materials that are vented to the atmosphere.
With syngas being a highly combustible material it is important to identify potential hazards posed in IGCC systems and find ways to improve the safe handling of this and other potentially hazardous gases in power generation systems.
In accordance with common practice, the various described device features are not drawn to scale, but are drawn to emphasize specific features relevant to the invention. Like reference characters denote like elements throughout the figures and text.